The present invention relates to a foamable silicone rubber composition capable of giving a cured silicone rubber having a cellular structure. More particularly, the invention relates to a foamable silicone rubber composition capable of giving a cured cellular silicone rubber body having excellent mechanical properties and good stability retaining the initial dimensions and forms even at a high temperature so as to be useful as a fire-resistant joint-sealing member in buildings and the like. The invention also relates to a cellular silicone rubber body obtained by foaming and curing the foamable silicone rubber composition.
The most traditional material used as a fire-resistant joint-sealing member in buildings is an asbestos-based cellular body which is recognized to satisfy the statutory standard for fire-resistant mate-rials in buildings. This traditional material, however, is destined to be entirely banned in the near future in view of the carcinogenicity of asbestos to cause lung cancers. Accordingly, it is eagerly desired to develop a substitute for the asbestos-based cellular body suitable as a fire-resistant joint-sealing member in buildings without the problems on the human health.
Various proposals and attempts have been made hitherto for such a fire-resistant joint-sealing material and, among the various materials thus far proposed, silicone rubber-based ones are considered to be the most promising. Although silicone rubbers are absolutely harmless against not only human bodies but also any animals, the conventional silicone rubber-based materials are not quite satisfactory in respect of the dimensional stability at high temperatures when the material is exposed to fire. When a silicone rubber body is kept in an atmosphere at a temperature gradually increasing from room temperature, for example, the body gradually expands as the temperature is increased up to around 400.degree. C. and then causes rapid expansion or shrinkage in the range of 400.degree. to 600.degree. C. resulting in irregular changes in the dimensions of the rubber body while the dimensional change of the rubber body is toward shrinkage when the temperature is further increased above 600.degree. C. In the temperature range exceeding 400.degree. C., thermal cracking may take place in the molecular structure of the organopolysiloxane as the principal ingredient of the silicone rubber not only in the chemical bonds between the silicon atoms and the organic groups but also in the siloxane linkages to form low molecular-weight combustible gaseous decomposition products. Once such a gas is produced within the silicone rubber body, cracks are formed unavoidably inside the body while the cracks are enlarged along with the shrinkage of the rubber body per se. Needless to say, the cracks having substantial broadness permit flames to pass therethrough or lead to eventual falling of the joint-sealing member as a whole increasing the danger of fire spreading. Thus, these silicone rubber-based joint-sealing materials are desired to be improved in respect of the stability at high temperatures so that spreading of fire can be reliably prevented thereby.
Japanese Patent Publication 63-191841 teaches that a silicone rubber-based fire-resistant joint-sealing material for buildings can be imparted with improved fire resistance by the admixture of the silicone rubber composition with a platinum compound in an amount up to 2000 ppm by weight and a ceramic-forming agent in an amount of 5 to 45% by weight. The improvement obtained by this means is still not high enough so that, when the joint-sealing member is subjected to a high-level test for fire-proofness to withstand prolonged exposure to fire for 3 hours, formation of cracks is noted or interstices are formed between the sealing member and the substrate body of the building not to prevent fire spreading therethrough.